Many pourable food products, such as fruit juice, UHT milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material. A typical example of this type of package is the parallelepipedic package known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing a web of laminated packaging material.
The packaging material has a multilayer structure substantially comprising a base layer of fibrous material, e.g. paper, and a number of layers of heat-sealable polymeric material, e.g. polyethylene films, covering both sides of the base layer. In the case of aseptic packages for long-storage products the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-sealable plastic material, and is in turn covered with another layer of heat-sealable polymeric material forming the inner face of the package eventually contacting the food product.
Typically packages made from such packaging material are produced in fully automatic packaging machines. In these machines the packaging material is sterilized, e.g. by applying a chemical sterilizing agent such as a hydrogen peroxide solution which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating. After sterilization the web of packaging material is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube. The tube is fed continuously in a first vertical direction, is filled with the sterilized or sterile-processed food product, and is gripped at equally spaced cross sections by pairs of jaws. More specifically, the pairs of jaws act cyclically and successively on the tube, and heat seal the packaging material of the tube to form a continuous row of cushion-shaped packages connected to one another by respective transverse sealing zones, extending in a second direction which is perpendicular to said first direction.
The cushion-shaped packages are separated by cutting the relative transverse sealing zones, and are then fed to a final folding station where they are folded mechanically into the finished parallelepipedic shape.
In an alternative to the above described technique, the packaging material may be cut into blanks. In this case a blank is firstly erected to form a sleeve which is sealed at a first end. Then the package is sterilized. Afterwards, the sleeve is filled with the pourable product through its open second end, and the second end is sealed, so as to complete the formation of the package.
In both cases, the packaging material in which the layer of barrier material comprises an electrically conductive material, is normally heat sealed by a so-called induction heat-sealing process, in which, eddy currents are induced in the aluminium layer, resulting in a localized heating and thus melting the heat-sealable polymeric material locally.
More specifically, in induction heat sealing, the sealing device substantially comprises an inductor powered by a high-frequency current generator. The inductor is a coil made of electrically conductive material and the generated electromagnetic field interacts with the aluminium layer in the packaging material to induce eddy currents in it and heat it to the necessary sealing temperature. The coil is typically made of e.g. a metallic material such as e.g. copper.
In case that packages are formed starting from a tube of packaging material, the sealing device is fitted to a first jaw. The other jaw, known as the anvil, comprises a counter-sealing element fitted with pressure pads made of elastomeric material, and which cooperate with the sealing device to heat seal the tube along a relative transverse sealing zone. More specifically, the sealing device locally melts the two layers of heat-sealable polymeric material gripped between the jaws.
Furthermore, the anvil may be arranged with a cutting element. In particular, the cutting element may slide towards and away from the sealing device of the sealing jaw along a third direction orthogonal to first and second direction such that it cuts the continuous row of packages into individual packages in accordance with the previous description.
In case that packages are formed starting from blanks of packaging material, the sealing device may be fitted to a jaw of a packaging machine.
A known sealing device for use in both cases substantially comprises an inductor coil having two sealing surfaces. The inductor coil is partly encapsulated in a supporting body having at least the sealing surfaces exposed on an outer surface of the supporting body, for cooperation with the packaging material during the formation of packages. The power connections are also exposed outside the supporting body. One or more inserts made of magnetic flux-concentrating material, e.g. a composite material comprising ferrite, is arranged inside the supporting body, close to the inductor coil, for guiding the electromagnetic field.
Each sealing surface also comprises a ridge which is intended to cooperate with the packaging material and increase the pressure thereon, so causing the fusion of the melted plastic material of the packaging material in the sealing area.
Typically, the sealing device, and therefore also the inductor coil, is exposed to high temperatures, high pressures and hydrogen peroxide in the packaging machine. This combination creates an aggressive environment which in certain applications causes rapid corrosion and wear of the inductor coil. Consequently, the sealing devices have to be replaced regularly, each replacement causing a stop in the production of packages.